As an active form of vitamin D-3, 1 alpha,25-(OH)(2)D-3 has a positive therapeutical effect on osteoporosis. However, 1 alpha,25-(OH)(2)D-3 can not only promote the osteoclastogenesis, but also inhibit the proliferation of osteoclast precursors (OCPs). Autophagy is regulated by 1 alpha,25-(OH)(2)D-3 and is considered to promote the osteoclastogenesis. Nevertheless, the role of 1 alpha,25-(OH)(2)D-3 in OCPs' autophagy remains unknown. Our study aims to explore whether the effect of 1 alpha,25-(OH)(2)D-3 on osteoclastogenesis is related to its regulation in autophagy. The results showed that 1 alpha,25-(OH)(2)D-3 exhibited a direct inhibitory effect on the autophagy activity and the proliferation of OCPs derived from bone marrow-derived macrophages (BMMs), which was reversed by the overexpression of autophagy-related gene. In presence of RANKL, the autophagy capacity of OCPs and the differentiation from OCPs into mature osteoclasts were significantly enhanced by 1 alpha,25-(OH)(2)D-3, while the suppression of autophagy with spautin-1 or 3-MA downregulated the osteoclastogenesis capacity. In summary, 1 alpha,25-(OH)(2)D-3 can directly suppress OCPs autophagy, which negatively regulates the proliferation of OCPs without RANKL. 1 alpha,25-(OH)(2)D-3 can indirectly upregulate the autophagy response of OCPs, thereby enhancing the osteoclasts formation in presence of RANKL. Therefore, our study found that 1 alpha,25-(OH)(2)D-3 had a dual effect on osteoclastogenesis by regulating autophagy, suggesting that some drugs targeting autophagy may act as an effective supplement of 1 alpha,25-(OH)(2)D-3 in treating osteoporosis. (C) 2019 Elsevier Inc. All rights reserved.